The blades in a wind turbine are typically attached, at a blade root portion, to a rotor hub, either directly or through an extender.
The blades are typically made from composite materials: reinforcing fibres may be bonded to one another with a resin and, subsequently, cured to consolidate. Different kinds of fibres (e.g. glass, carbon, combinations) and resins (e.g. epoxy, polyester, vinyl ester) may be used in different forms (e.g. prepreg, fibre sheets). Such composite materials may not have the structural integrity to provide a secure fixing mechanism into which, for example, threaded bolts may be directly inserted. For this reason, it is known for wind turbine blades to comprise metal inserts or bushings which are embedded into the blade root in a direction parallel to that of the longitudinal axis of the blade. Studs, bolts or rods can thus be used in combination with these bushings to achieve a secure connection between the hub or extender and the blade. Such a connection must be able to withstand all the loads from the blade, i.e. bending moments due to wind pressure and to the weight and rotation of the blades. It is thus desirable for a blade to comprise metal inserts strongly bonded inside the blade root portion.
In general terms considering a structural design of the blades, two main processes are well-known for the manufacturing of composite blades: prepreg and infusion.
Prepreg is the abbreviation for “pre impregnation” and is the process where a fibre layer or fabric is impregnated with a resin to form a homogeneous precursor that is subsequently used to manufacture composite components. Typical curing temperatures for prepreg processes may range between 80° to 120° C. The moulds for prepreg processes should thus be made to withstand such temperature ranges.
In the infusion process, once the fibre layers are distributed, a resin is provided using vacuum and subsequently cured to obtain the composite components. Typical curing temperatures for infusion processes may range between 50° to 70° C. The moulds used for infusion processes may thus be cheaper. Furthermore, the infusion process uses slightly lower performance resins (in terms of strength) due to a low viscosity requirement for facilitating resin infusion. The infusion process is thus usually considered to be a cheaper process. However prepreg processes are normally considered as providing a more accurate fibre alignment, higher performance resins and low void content. Particularly in larger wind turbine blades, the use of prepreg for their manufacturing results in more expensive blades.
A problem related to the manufacture of blades by infusion is the fact that the metal inserts do not adhere too well to a blade root manufactured by infusion. In order to facilitate a proper adhesion between the inserts and the blade root, the inserts are thus usually adapted, e.g. they may comprise a wedge-shaped end for insertion or they may comprise roughness on its external surface. In some cases the inserts may be externally serrated. In some cases the inserts may be made longer than would be necessary to arrange the stud so as to have a larger adhesion surface.
It is thus an object of the present disclosure to provide improved methods of manufacturing wind turbine blades with higher mechanical properties and which are cost-effective.